516 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



of broken porcelain and heated to 1300 C. On absorbing 

 the carbonic anhydride by potash a continuous supply of 

 detonating gas was obtained; at the end of two hours, 25 

 to 30 cubic centimetres had been collected (loc. tit. p. 323). 

 Dissociation of carbonic anhydride. When dry carbonic 

 anhydride was passed through the same apparatus, an 

 explosive mixture of carbonic oxide and oxygen was 

 collected at the rate of 20 to 30 cubic centimetres per hour. 

 The combustion of carbonic oxide is therefore a reversible 

 process as shown by the dotted arrow in the equation : 



Deville's hot-cold tube (1864). In most cases of 

 dissociation the extent of the decomposition increases very 

 rapidly with the temperature. If a substance be strongly 

 heated and then suddenly chilled it is sometimes possible to 

 fix the products of dissociation in the larger proportions in 

 which they exist at higher temperatures ; on the other 

 hand, if the cooling is gradual they recombine and are only 

 retained in the small proportions which are natural at the 

 lower temperature at which recombination ceases. Deville 

 secured this strong heating and rapid cooling in his "hot- 

 cold " tube shown in Fig. 55 (Lessons on Dissociation, p. 64). 

 The outer tube of porcelain was heated to whiteness ; the 

 inner tube of brass was cooled by a current of water. With 

 this apparatus he demonstrated the decomposition of car- 

 bonic oxide, of sulphurous anhydride, and of hydrogen 

 chloride : 



2 CO ^ C +CO.; 



3 so 2 ^ s + 2 s6 3 



In the first case, carbon was deposited on the cold tube 

 and carbonic anhydride carried forward (Comptes rendus, 

 1864, 59, 875); in the second case, sulphide and sulphuric 

 anhydride were both deposited ; in the third case, chlorides 



